Optoelectronic devices are an increasingly central part of everyday life. Smart phones, computers, televisions, handheld electronics, radio-frequency ID tags (RFIDs), ‘smart’ appliances, photovoltaic devices, and more, include such optoelectronic devices. Examples of such devices include: displays, such as liquid crystal displays (LCDs) and organic light emitting diode displays (OLEDs); photovoltaic (PV) devices, including crystalline silicon, inorganic thin-film, and organic photovoltaic (OPV); and field-effect transistors (FETs), which are a key element in many electronic devices. The trend is to reduce the size and/or cost of these optoelectronic devices in order to enable widespread commercial adoption. Once low enough thresholds are reached for size and/or cost, such devices are expected to become nearly ubiquitous in everyday life.
In order to make these devices more cost-effective, techniques that allow high-throughput large-area manufacturing are needed to reduce the cost per unit device to reasonable levels. While optoelectronic devices are diverse, and thus the materials and manufacturing techniques involved vary quite a bit, there are a number of common elements in a variety of optoelectronic devices. Many such devices require the controlled transport of electrons and/or holes (i.e., electron vacancies) into or out of the device, in order to precisely control the flow (e.g., in FETs), separation (e.g., in PV), or recombination (e.g., in OLEDs) of such particles in the device, enabling the desired device properties. The materials used to enable such controlled flow of electrons or holes in a device are referred to as electron transport layers or hole transport layers (ETLs or HTLs), respectively. An ETL will allow the transport (flow, collection, or injection, depending on the device) of electrons, while blocking the transport of holes in a device, while a HTL will do the opposite.
While there are a variety of ETL and HTL materials used in the many various types and versions of optoelectronic devices in existence, many common transport layers are based upon metal oxide thin films. Metal oxide thin films have a number of advantages over alternative materials, such as thin polymer films and self-assembled monolayers (SAMs). Metal oxide thin films are relatively well-studied and understood materials and are generally physically, thermally, and chemically robust. The variety of metals that form useable oxides ensure a broad range of such device-important physical properties, such as n-type or p-type material, work function, conductivity, electron/hole mobility, optical transparency and reflectivity. In contrast to metal oxides, thin polymer film transport layers are generally much less well studied and understood materials, often have low mobilities, which require very thin films (˜5 nm) to ensure adequate performance, and as such often have poor physical robustness. Additionally, thin polymer films are generally much less thermally stable than metal oxides. Similarly, SAM transport layers are poorly studied materials, and are not currently well understood. Their monolayer nature ensures very fragile films with high potential for pinholes/shorts and often exhibit poor thermal and chemical stability.
Metal oxide thin films can be produced via a variety of techniques, including: sputtering, chemical vapour deposition (CVD), pulsed-laser deposition (PLD), atomic layer deposition (ALD), thermal evaporation, and sol-gel chemistry methods. These techniques share a common disadvantage in that they either require a vacuum based process to enable the film deposition or they require subjecting materials to high temperatures for extended time periods. Vacuum-based process significantly increases the time and cost of depositing metal oxide thin films, as samples are pumped down to the desired vacuum levels, the deposition performed, and then the samples returned to atmospheric pressure levels. High temperature techniques, which often require temperatures in excess of 300° C., add significant cost due to the high energy demands on obtaining and maintaining such temperatures. Furthermore, such high temperatures significantly limit the range of substrates that can be used. For example, temperatures above 150° C. for extended periods prevent the use of many polymer foils, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), often used in high-throughput roll-to-roll manufacturing lines. Additionally, elevated temperatures tend to cause damage to any other underlying layers exposed to the high temperatures. Meanwhile, nanoparticle techniques produce materials with diminished transport and hole blocking characteristics as compared metal oxide thin films produced using the sol-gel or vacuum deposition methods, and their use is complicated by wetting and aggregation issues that hinder large-scale production.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.